When a television receiver displays images of a relatively low refresh rate, such as a PAL (Phase Alternate Line) signal of 50 fields per second, a phenomenon called screen flicker occurs.
By doubling the field frequency from 50 fields per second to 100 fields per second (hereinafter, this conversion technique is called a field doubling technique), the television receiver can reduce the screen flicker.
FIG. 7 shows a television receiver 100 adopting the above field doubling technique. For example, an input video signal S1 of 50 fields per second, such as a PAL signal, is input to a field doubling circuit 101.
A double-speed conversion unit 102 of the field doubling circuit 101 converts a scanning method of the input video signal S1 from interlace scanning (interlace scanning) to progressive scanning (noninterlace scanning) to create an intermediate video signal S2 of 50 frames per second, and writes this in an image memory 103. Then the double-speed conversion unit 102 reads the intermediate video signal S2 from the image memory 103 at a double speed of writing to double the field frequency of the intermediate video signal S2 to 100 fields per second, changes the scanning method from the progressive scanning to the interlace scanning, and inputs the created double-speed video signal S3 to a CRT (Cathode Ray Tube) 104.
At this time, the CRT 104 is receiving a horizontal/vertical serrate wave S4 of 100 fields per second from a horizontal/vertical deflection circuit 105, and displays the double-speed video signal S3 at 100 fields per second based on the horizontal/vertical serrate wave S4.
FIG. 8 shows a positional relation between fields and scanning lines of a video signal before and after being subjected to the above-described field double-speed conversion. A horizontal axis represents time and a vertical axis represents a vertical direction of a screen. Each white circle represents a scanning line. Reference numerals 1a, 1b, 2a, 2b, . . . shown in FIG. 8(A) represent field numbers, numerals 1, 2, . . . represent frame numbers, and a and b represent an odd field and an even field.
The double-speed conversion unit 102 (FIG. 7) of the field doubling circuit 101 first doubles the number of scanning lines of a field 1a of the input video signal S1 comprising an interlace image shown in FIG. 8(A) (progressive conversion), thereby creating a frame 1a shown in FIG. 8(B). Similarly, the double-speed conversion unit 102 sequentially performs the progressive conversion on the fields 1b, 2a, 2b, . . . of the input video signal S1 to create frames 1b, 2a, 2b, . . . , thereby creating an intermediate video signal S2 of a progressive image of 50 frames per second.
The double-speed conversion unit 102 writes the frame 1a shown in FIG. 8(B) in the image memory 103, and reads it every other scanning line. At this time, the double-speed conversion unit 102 first reads the odd-numbered scanning lines of the frame 1a to create a field 1a shown in FIG. 8(C). Then 1/100 second later, the unit 102 reads the even-numbered scanning lines of the frame 1a to create a field 1a′ shown in FIG. 8(C). In this way, the double-speed conversion and the interlace conversion can be performed.
Similarly, the double-speed conversion unit 102 sequentially performs the double-speed conversion and the interlace conversion on the frames 1b, 2a, 2b, . . . of the intermediate video signal S2 to create fields 1b, 1b′, 2a, 2a′, 2b, 2b′, . . . , thereby creating a double-speed video signal S3 of an interlace image of 100 fields per second.
As described above, the double-speed conversion unit 102 of the field doubling circuit 101 doubles the field frequency of the input video signal S1 from 50 fields per second to 100 fields per second, so as to suppress screen flicker.
The fields 1a and 1a′ of the double-speed video signal S3 shown in FIG. 8(C) are both fields created from the field 1a of the input video signal Si shown in FIG. 8(A). Similarly, the fields 1b and 1b′, the fields 2a and 2a′, fields 2b and 2b′, . . . of the double-speed video signal S3 are fields created from the same fields of the input video signal S1.
A movement of an image subjected to the double-speed conversion will be described with reference to FIG. 9. In FIG. 9, a vertical axis represents time and a horizontal axis represents a horizontal direction of a screen. An entity 110 on an image moves from the left to the right of the screen with time.
In the input video signal S1 shown in FIG. 9(A), the entity 110 smoothly moves in the right direction between fields. In the double-speed video signal S3 shown in FIG. 9(B), on the other hand, the entity 110 is displayed at the same position in the fields 1a and 1a′ because the fields are created from the same field. Then since the next field 1b is created from a different field, the entity 110 moves greatly from the field 1a′ to the field 1b.
Similarly, since the fields 1b and 1b′, the fields 2a and 2a′, the fields 2b and 2b′ are created from the same fields, the entity 110 moves greatly from the field 1b′ to the field 2a, and from the field 2a′ to the field 2b. 
Therefore, the double-speed video signal S3 subjected to the double-speed conversion has a problem in that moving image cannot be displayed smoothly.
In a case of displaying a cinema film composed of still images of 24 frames per second, on a normal television receiver, the cinema film is converted into a video signal with so-called telecine conversion.
In this case, as shown in FIG. 10(A), in an input video signal S1 of a film material created by performing the telecine conversion on the cinema film, fields 1a and 1b, fields 2a and 2b, . . . are created from the same segments of the film with the telecine conversion. Therefore, in a double-speed video signal S3 (FIG. 10(B)) created by performing the double-speed conversion on the input video signal S1, four fields 1a, 1a′, 1b and 1b′ are created from the same segment. Similarly, four fields 2a, 2a′, 2b, 2b′ are created from the same segment.
Therefore, in the double-speed video signal S3 shown in FIG. 10(B), an entity 110 is displayed at the same place in the four fields 1a to 1b′, and moves greatly when shifting to the next field 2a. 
As a result, the double-speed video signal S3 created by performing the double-speed conversion on the input video signal S1 for the film material has a problem in that a movement of an moving image is considerably non-smooth.
To solve the above non-smooth movement problem of a double-speed video signal, a motion compensation method has been proposed to realize smooth movement between fields by detecting motion vectors of an image and shifting the image with the motion vectors (for example, refer to patent reference 1).
In the motion compensation method, a motion vector of an image is detected on a pixel basis or on a block basis between the field 1a′ and the field 1b′, which is one frame later, of the double-speed video signal S3 shown in FIG. 11(A) with, for example, a block matching method. The detected motion vector is taken as A.
Then, as shown in FIG. 11(B), the pixel of the field 1a′ from which the movement of the block is detected is shifted by A×½. Similarly, a motion vector B of the image between the field 1b′ and the field 2a′, which is one frame later, is detected, and the pixel of the field 1b′ from which the movement of the block is shifted by B×½. By shifting the image with motion vectors detected as described above, the image can be moved smoothly between fields as shown in FIG. 11(C).
In a case where the double-speed video signal S3 is a film material created with the telecine conversion, a motion vector of the image in the double-speed video signal S3 shown in FIG. 12(A) is detected on a block basis between the field 1a and the field 2a that is two frames later, with the block matching method. The detected motion vector is taken as A.
Then as shown in FIG. 12(B), the pixels of the fields 1a′, 1b, and 1b′ from which the movement of the block is detected are shifted by A×¼, A× 2/4, and A×¾, respectively. By shifting the image with motion vectors detected in this way, the image can be moved smoothly between fields as shown in FIG. 12(C).
Patent Reference JP-A-H10-501953
Actually, in many cases, a luminance level and color vary as an image moves. In such cases, the above-described motion compensation method causes a little unnaturalness in a compensated image. This is a problem.
Further, motion vectors which are detected with the above-described block matching method are not always appropriate and motion vectors that are greatly different from actual movement of an image may be detected. For example, such erroneous detection of motion vectors is easily caused in a case where two movements exists in a block, such as a case where background and foreground moves in different directions, where an image is turning, or where an image is zoomed or deformed.
Since the above-described motion compensation method performs motion compensation by ¾, at maximum, of a motion vector (fields 1b′ and 2b′ of FIG. 12), the unnaturalness of a compensated image becomes remarkable when a motion vector is detected erroneously. This is also a problem.
Further, as shown in FIG. 13, in a case where such a dynamic movement as to exceed a search range of the block matching method exists in an image, a correct motion vector cannot be obtained, which causes unnaturalness in a compensated image. This is also a problem.